A sintered diamond material used for conventional diamond tools is obtained using a metal such as cobalt (Co) and a ceramic such as silicon carbide (SIC) as a sintering aid and a binder. Further, Japanese Patent Laying-Open No. 4-074766 (Patent Document 1) and Japanese Patent Laying-Open No. 4-114966 (Patent Document 2), for example, disclose a method using carbonates as sintering aids. According to these documents, a sintered diamond material is obtained by sintering diamond powder along with a sintering aid and a binder under stable high-pressure and high-temperature conditions in which diamond is thermodynamically stable (generally, a pressure of 5 to 8 GPa and a temperature of 1300 to 2200° C.). Naturally occurring polycrystalline diamond bodies (carbonado and ballas) are also known, and some of them are used for drill bits. These polycrystalline diamond bodies, however, are not much used for industrial purposes, since they vary significantly in material quality, and can only be found in limited quantities.
A polycrystalline diamond body obtained using a sintering aid contains the sintering aid, which may act as a catalyst promoting graphitization of diamond. As a result, the heat resistance of the resulting polycrystalline diamond body deteriorates. Further, when heat is applied to the polycrystalline diamond body, fine cracks tend to develop due to a difference in thermal expansion between the catalyst and the diamond. As a result, the mechanical properties of the polycrystalline diamond body deteriorate.
Polycrystalline diamond bodies are also known from which the metal present at grain boundaries of diamond particles has been removed to improve the heat resistance. Although this method improves the heat-resistant temperature to about 1200° C., the polycrystalline body becomes porous and thus, has further decreased strength. A polycrystalline diamond body obtained using SiC as a binder has high heat resistance, however, it has low strength because diamond particles are not bonded together.
A method is also known in which non-diamond carbon such as graphite or amorphous carbon is directly converted into diamond at an ultra-high pressure and a high pressure, without using a catalyst and/or a solvent, and sintered simultaneously (direct conversion and sintering method). J. Chem. Phys., 38 (1963) pp. 631-643 (Non-Patent Document 1), Japan. J. Appl. Phys., 11 (1972) pp. 578-590 (Non-Patent Document 2), and Nature 259 (1976) p. 38 (Non-Patent Document 3), for example, show that a polycrystalline diamond body is obtained using graphite as a starting material under an ultra-high pressure of 14 to 18 GPa and a high temperature of 3000 K or more.
However, in the production of a polycrystalline diamond body according to Non-Patent Documents 1, 2, and 3, a method of heating by direct current passage is used in which conductive non-diamond carbon such as graphite is heated by directly passing current therethrough. The polycrystalline diamond body thus obtained contains remaining non-diamond carbon graphite, and also has a nonuniform crystal grain size of diamond. As a result, the polycrystalline diamond body has poor hardness and strength.
In order to improve the hardness and strength, New Diamond and Frontier Carbon Technology, 14 (2004) p. 313 (Non-Patent Document 4) and SEI Technical Review 165 (2004) p. 68 (Non-Patent Document 5) show a method for obtaining a dense and high-purity polycrystalline diamond body by a direct conversion and sintering method in which high-purity graphite as a raw material is indirectly heated at an ultra-high pressure of 12 GPa or more and a high temperature of 2200° C. or more.